Recently, in the field of information recording, a wide variety of recording materials have been researched, developed and utilized in accordance with diversification of information and expansion of the needs. Among them, thermal recording materials have advantages that (1) it can record images simply by a heating process only and (2) a mechanism of a required device is simple so that the device can be downsized easily and the recording materials are inexpensive and easy to be handled. Therefore, the thermal recording materials are used in various fields, such as a field of information processing (an output such as a desk calculator, or a computer), a field of recorders for medical measurements, a field of low-speed and high speed facsimiles, a field of automatic machines for tickets (train tickets, bus tickets, entrance tickets, etc.), a field of heat-sensitive copying machines, and a field of labels in a POS system.
For these thermal recording materials, it is required to develop color rapidly with a high density and to have high fastness of a developed image and background. For the means for developing the color rapidly with a high density (enhancing sensitivity), it has been studied to add a developer having a low melting point, or a material, as a sensitizer, that induces melting point depression by melting with a developer or a leuco dye, which is an electron-donating colorless dye. Basically, lowering the melting point of the developer and the sensitizer is fairly efficient for enhancing sensitivity, but it has a disadvantage that background fog (reduction of whiteness) is produced at a low temperature in contrast to improvement of the sensitivity, because the color developing temperature decreases.
In the process for producing the thermal recording material, a dispersion that contains a heat-sensitive dye and a developer is prepared. For other means for enhancing sensitivity of the thermal recording material, various techniques have been proposed in which a leuco dye is microparticulated so that the sensitivity of the thermal recording material increases. For example, use of polyvinyl alcohol containing a sulfonic group (patent document 1), use of polyvinyl alcohol containing a carboxyl group (patent document 2), and use of a resin composition of polyvinyl alcohol and an olefin/maleic acid copolymer (patent document 3), as a dispersant for a heat-sensitive dye or the like, have been proposed. In addition, in order to improve the whiteness and the background fog under high humidity, it has been proposed that a dispersion containing a heat-sensitive dye is subjected to heat-treatment at 40° C. or more for 3 hours or more (patent document 4).
However, these past proposals still have some problems. The problems are that the premature graying of the dispersion is occurred more drastically as downsizing of the size of the dispersed particles (premature graying means the phenomenon that a dispersion becomes darker as downsizing of dye particles), that the viscosity of the dispersion easily becomes high with time because the viscosity stability when the obtained dispersion is left to stand is insufficient, and that when the dispersion is heat-treated to improve the whiteness and the background fog under high-humidity environment, the dispersion state becomes unstable, resulting in reaggregation, viscosity increase, gelation, and the like. In the conventional art, a dispersion that contains particles with a small particle diameter, shows hardly any premature graying, and exhibits excellent dispersion stability and storage stability when heated has not been obtained yet.    Patent document 1: JP-S58-179691A    Patent document 2: JP-H8-48076A    Patent document 3: JP-H11-321103A    Patent document 4: JP2004-359802A